The present invention relates generally to light-emitting diode array power supplies and more particularly to such supplies that use an operational amplifier to regulate a constant current source. This application is related to my co-pending patent application entitled "Precision Reference Current Generator", Ser. No. 07/944,852.
In electrophotographic ("EP") printers images are formed by selectively discharging a photoreceptor surface and then attracting oppositely charged toner to the remaining charged surface. One means for discharging the photoreceptor is with an array of light-emitting diodes ("LEDs"), where the array is comprised of individual LEDs spanning the width of the photoreceptor surface. The quality of the image is directly influenced by the LED's ability to provide consistent and uniform light to the photoreceptor. The consistency and uniformity of the light produced by the LEDs is a function of the stability of the current supplied to each LED by its respective current source. However, the physical implementation of the array and the operating conditions within which it operates each contribute to variations in the current supplied to the LEDs.
Prior Thayer et al. U.S. Pat. No. 5,099,192 ("Thayer"), which is incorporated herein by reference, teaches a power supply having a multiple number, such as five, of parallel current sources for biasing each LED in the print head. Each current source includes a P-channel driver FET in series with a P-channel control FET, and referred to as an "output-control pair." The driver FET has a source coupled to the positive power supply, a gate for receiving a reference voltage, and a drain. The control FET has a source coupled to the drain of the driver FET, a gate for receiving a control voltage, and a drain coupled to an LED. The drains of each control FET being connected in parallel increases effective channel width. The digital control signal at the gate of each control FET selectively enables or disables the respective output current from each output-control pair that is delivered to the LED. This allows the total current delivered to each LED to be controlled and matched to the characteristics of the LED being driven. However, once the effective width is established it remains fixed unless a new programming pattern is applied.
Thayer et al. describes two primary factors that contribute to the non-uniformity in the light output of the LEDs and teaches solutions to both. The first factor is the variation in the light output of the LEDs as a function of current between different wafer lots. These variations can be largely eliminated by sorting the LEDs according to their light efficiency. In addition to the variations within the LEDs, the current output of the power supplies can also vary from power supply to power supply due to processing variations. In Thayer et al. these variations can be partially compensated for by appropriately sizing and selectively enabling the individual current outputs of the output-control pairs.
Under constant operating conditions the power supply in Thayer et al. provides a substantially constant current that is suitably matched to the particular LED being driven. However, the current output can fluctuate with changes in the operating voltage at the drain of the control FET, i.e. the output of each current source. No provision is made in Thayer et al. to modify the digital settings at the gates of the control FETs, or otherwise compensate the power supply, in response to fluctuations in the output operating voltage.
Due to physical and economic considerations, each current source output I.sub.1 through I.sub.N from the power supply is preferably multiplexed, between several LEDs, e.g., four LEDs per current source, as shown in FIG. 1. Multiplexing the current sources with multiplexers MUX.sub.1 through MUX.sub.N reduces the total number of current sources that are required to drive all of the LEDs. Three multiplexers and four LEDs per multiplexer are shown in FIG. 1, but any number of multiplexers and LEDs can be used. Although the multiplexers are shown between the current sources and the LEDS, an equivalent circuit results from locating the multiplexers between the LEDS and the current return path, i.e., GND. In either case, the introduction of the multiplexing circuitry causes the voltage at the output of the current supply to fluctuate, which in turn affects the current source's ability to provide a constant current. In addition, there is a changing voltage associated with the current return path due to the changing number of LEDs injecting current into the finite impedance of the GND return path. This causes further fluctuation in the voltage at the output of the current source with a corresponding change in output current.
What is desired is a power supply for driving LEDs having multiple switched current outputs in which the current produced thereby is precisely controlled and is independent of fluctuations in the operating voltage at the current output coupled to the LEDs.